Fiber optic filters are well known in the art, and may be constructed using a combination of optical fiber and gratings. Using fiber of the previously described type, there are several techniques for creating fiber optic gratings. The earliest type of fiber grating-based filters involved gratings external to the fiber core, which were placed in the vicinity of the cladding as described in the publication “A single mode fiber evanescent grating reflector” by Sorin and Shaw in the Journal of Lightwave Technology LT-3:1041–1045 (1985), and in the U.S. patents by Sorin U.S. Pat. No. 4,986,624, Schmadel U.S. Pat. No. 4,268,116, and Ishikawa U.S. Pat. No. 4,622,663. All of these disclose periodic gratings which operate in the evanescent cladding area proximal to the core of the fiber, yet maintain a separation from the core. A second class of filters involve internal gratings fabricated within the optical fiber itself. One technique involves the creation of an in-fiber grating through the introduction of modulations of core refractive index, wherein these modulations are placed along periodic spatial intervals for the duration of the filter. In-core fiber gratings were discovered by Hill et al and published as “Photosensitivity in optical fiber waveguides: Application to reflected filter fabrication” in Applied Physics Letters 32:647–649 (1978). These gratings were written internally by interfering two counter propagating electromagnetic waves within the fiber core, one of which was produced from reflection of the first from the fiber endface. However, in-core gratings remained a curiosity until the work of Meltz et al in the late 1980s, who showed how to write them externally by the split-interferometer method involving side-illumination of the fiber core by two interfering beams produced by a laser as described in the publication “Formation of Bragg gratings in optical fibers by a transverse holographic method” in Optics Letters 14:823–825 (1989). U.S. patents Digiovanm U.S. Pat. No. 5,237,576 and Glenn U.S. Pat. No. 5,048,913, also disclose Bragg gratings, a class of grating for which the grating structure comprises a periodic modulation of the index of refraction over the extent of the grating. Within this class of in-fiber gratings, most of the art is directed to in-fiber gratings having the Bragg plane of refractive index modulation perpendicular to the principal axis of the core of the fiber optic cable. A new class of grating involves in-fiber gratings with an angular offset in the plane of refractive index modulation. This type of angled grating is referred to as a mode-converting two-mode grating, and, with properly chosen angle, has the property of converting fundamental-mode power into second-mode power and visa versa. Whether internal or external, both types of gratings can be fabricated as short-period gratings, or long-period gratings. Short-period gratings reflect the filtered wavelength into a counter-propagating mode, and, for silica based optical fibers, have refractive index modulations with periodicity on the order of a third of the wavelength being filtered. Long-period gratings have this modulation period much longer than the filtered wavelength, and convert the energy of one mode into another mode propagating in the same direction, i.e., a co-propagating mode, as described in the publication “Efficient mode conversion in telecommunication fibre using externally written gratings” by Hill et al in Electronics Letters 26:1270–1272 (1990). The grating comprises a periodic variation in the index of refraction in the principal axis of the core of the fiber, such variation comprising a modulation on the order of 0.1% of the refractive index of the core, and having a period associated with either short or long-period gratings, as will be described later.
Tunable fiber-optic filters can be produced in a variety of ways. FIG. 1 shows a fiber optic cable 12 having a core 14 which has a grating 16 written over an extent Lg 18. The pitch of the grating 16 may be fixed or variable, and may be incrementally varied by changing the temperature of the fiber in the region of the grating. Germanium doped silicon has a coefficient of thermal expansion of 10 ppm/° C. Alternatively, the fiber may be placed in a variable tension, and this tension causes a shift in wavelength, as in the case of U.S. Pat. No. 6,597,822 by Moslehi et al. In U.S. Pat. No. 4,968,623 by Sorin, the pitch of a grating is varied by applying a proximal grating mounted on a disk and rotating it to vary the apparent pitch experienced by the proximal fiber. In U.S. Pat. No. 6,011,881 by Moslehi et al, a method for tuning a fiber optic grating coupled to a variable index material is disclosed. U.S. Pat. No. 6,411,746 by Chamberlain et al discloses a method for tuning an optical filter comprising grating coupled to a heater.
It is desired to provide a tunable fiber optic filter where the strain of a grating is varied through the expansion of an expander having a large extent which is coupled to the smaller extent of a Bragg grating. It is also desired to provide a magnetically tunable fiber-optic filter where the extent of an expander is the same as, or greater than, the extent of the fiber Bragg grating.
There are many materials known for its magnetostrictive properties, and among these materials the material of choice is Terfenol™ (Terfenol is a registered trademark of Etrema Products www.etrema.com, and Terfenol™ information is available at www.etrema-usa.com). Other magnetostrictive materials which change length in response to an externally applied magnetic field are KelvinAll®, Terbium-Dysprosium, and Terbium-Dysprosium-Zinc. FIG. 13 shows the magnetostrictive property of Tb0.3 Dy0.7 Fe1.9 which is commonly known by the tradename Terfenol-D™. Until recently, magnetostrictive materials typically produced lower strains than piezoceramic and electrostrictive materials. The graph 230 shows the magnetostriction of Terfonol-D varying from 0 to approximately 1800 ppm over the range 0 to 2000 Oersteds of magnetic field, which is on the order of 40 times greater magnetostriction than previous magnetostrictive materials, and a facter of 10 greater than piezoceramic devices. The graph 230 has a quadratic response for small fields (below 200 Oe), a quasi-linear response for fields from 200 Oe to 600 Oe, and a saturating region for fields in excess of 600 Oe. These points are approximate, and may vary depending on the particular material used.